1. Field of the Invention
The present invention relates to the preparation and use of solids-free fluids for oil and gas drilling, completion and workover operations. More particularly, the invention relates to new calcium-free fluids which may be used as completion, packer and perforating media in oil and gas drilling and completion operations when formations have high carbonate and/or high sulfate ion concentrations.
2. Description of the Prior Art
Special fluids known as drilling fluids are used in the drilling, completion, and workover of oil and gas wells. These fluids ideally perform the following functions: transport drill cuttings or solids debris to the surface suspend cuttings and solids in lost circulation zones counteract formation pressure; maintain borehole stability: cool and lubricate downhole equipment; aid the suspension of tool string and casing; minimize corrosion; and minimize damage to formation permeability.
Use of these drilling fluids has greatly increased the efficiency of operations at the well. However, problems with certain applications of these fluids have been encountered. For example, when used in completion operations, these fluids leave a deposit of acid-insoluble filter cake in the bore hole which blocks production and is difficult to remove. Further, use of these fluids may permit entry of fresh water mud filtrates which can promote the hydration of naturally occurring clay materials which swell in volume and restrict permeability. Finally, because of the high alkalinity of many of these fluids, precipitation of insoluble hydroxides occurs along the filtration path, impeding production. These problems have been partially overcome by underreaming or acidizing of the bore hole if the damage is not severe.
In recent years, however, specialized solids-free completion and workover fluids have been developed to help prevent this type of damage to formation permeability. These solids-free fluids are placed across the production zone during completion and workover operations performing the same functions as drilling fluids but minimizing formation damage. These solids-free completion fluids comprise concentrated salt-water solutions in the density range of about 10 to 21 pounds per gallon ("lb/gal" or "ppg") and may be used as perforation, gravel pack, packer, and workover media. Examples of these solutions include aqueous solutions of alkali and alkaline earth metal and zinc halides such as sodium chloride, sodium bromide, calcium chloride, calcium bromide, zinc bromide or mixtures thereof.
As disclosed in 1964 in U.S. Pat. No. 3,126,950 ("'950"), concentrated solutions of zinc chloride and/or calcium chloride can be prepared and used as well completion fluids up to a density of about 17 lb/gal. As noted in the '950 patent, however, zinc chloride/calcium chloride solutions with densities greater than 14 lb/gal. have high ferrous metal corrosion rates and therefore cannot practically be used with most well and surface equipment. Further, solutions with densities in the 14 lb/gal. range are not highly effective for deep well drilling. As a result of these limitations, these completion fluids did not receive strong acceptance in the oil and gas industry.
Other solids-free completion fluids have been better received. These fluids comprise calcium bromide, calcium chloride, and water and have densities up to 15.1 lb/gal. See Plonka, "New Bromide Packer Fluids Cut Corrosive Problems," World Oil, April 1972, and Paul and Plonka "Solids-Free Completion Fluids Maintain Formation Permeability," SPE 4655, Las Vegas, Sept. 30-Oct. 3, 1973. Unlike the fluids in the '950 patent, calcium bromide/calcium chloride fluids have very low corrosion rates, which can be further reduced with the addition of suitable corrosion inhibitors. Density limitations (15.1 lb/gal limit) and high crystallization point temperatures (68.degree. F.) of the calcium bromide/calcium chloride fluids, however, have made these fluids less than ideal for use in completion operations. Therefore demands for other new solids-free completion fluids have continued.
Another new system of completion fluids in the density range of 15.0 to 19.2 lb/gal was disclosed in 1981 in U.S. Pat. No. 4,292,183, ("'183"). The '183 patent teaches mixtures of zinc bromide, calcium bromide, calcium chloride, and water which contain corrosion inhibitors capable of reducing the corrosion rate of mild steel coupons to less than 10 mpy at 250.degree. F.
Although the introduction of these various new completion fluids have helped resolve many of the difficulties encountered in completion and workover operations, problems still remain. For example, use of completion fluids with significant zinc and calcium ion concentrations in subterranean wells containing carbonate or carbon dioxide result in precipitation of calcium and zinc carbonates. Further, it has been reported by Shaughnessy, et al. in "Workover Fluids for Prudhoe Bay," February-July 1977 that the mixing of calcium chloride workover fluids with formation brines under certain conditions (i.e., at a pressure of 5000 psi and a temperature of 220.degree. F.) can lead to the precipitation of calcium carbonate within reservoir rock and, therefore, to formation damage. These problems have been partially resolved by utilizing sodium bromide completion and workover fluids in place of calcium containing solutions. However, sodium bromide solutions can only be used in shallow wells where high formation pressures are not encountered. Further, more recently, carbon dioxide or carbonate containing wells have been discovered which require drilling and completion fluids with fluid densities of at least 14-20 lb/gal, density ranges which are well above those of sodium bromide.
The prior art generally describes solutions formulated from zinc and calcium bromides and various alkali metal bromides. However, the art has not heretofore taught the combination in calcium-free solutions of zinc bromide with one or more alkali metal bromides within the high density range (i.e.. up to about 20.5 pounds per gallon. Borchardt, et al. U.S. Pat. No. 4,554,081 is particularly concerned with brine additives for reducing fluid loss. The patentee teaches that such brines "typically contain KCl, NaCl, CaCl.sub.2, NaBr, CaBr.sub.2, ZnCl.sub.2 and ZnBr.sub.2, or combinations of such salts . . . " (Column 1, lines 47-50). The patent further notes that the salt(s) may be dissolved in water such that the resulting densities range from about 9.0 to 21.5 pounds per gallon. However, the patent does not recognize the dangers posed by using high density calcium containing fluids, and it discloses no high density calcium free fluids at all. Thus, in Example II and Table I there are disclosed 11.7 ppg calcium chloride solutions and 10.8 ppg sodium bromide solutions. The only mixed salt solutions all contain calcium salts (e.g. CaBr.sub.2 /CaCl.sub.2 and ZnBr.sub.2 /CaBr.sub.2). Examples III and IV are to the same effect. No combination of zinc bromide and one or more alkali metal bromide solutions is anywhere exemplified or otherwise disclosed by Borchardt, et al.
Heilweil U.S. Pat. No 4,609,476 and Heilweil, et al. U.S. Pat. No. 4,619,773 each describe high temperature stable aqueous brines. Heilweil notes the problems posed by brines containing divalent salts of calcium and zinc, and suggests that the utility of sodium bromide brines as an alternative has been limited by the lack of proper viscosifying agents (Column 2, lines 24-34.) The patent discusses brines containing densifying salts such as LiCl, NaCl, CaCl.sub.2, CaBr.sub.2, ZnBr.sub.2 and their mixtures (Column 6, lines 5-6). The only fluids exemplified in the patent are NaBr and CaBr.sub.2 fluids (see columns 5 and 6 of the reference). As in the case of Borchardt, et al., there is no hint of combining zinc bromide and one or more alkali metal bromides in a calcium-free high density fluid. This omission is particularly significant in the case of Heilweil because of its appreciation of the problems posed by divalent brines.
Heilweil et al. also note the problems posed by divalent calcium and zinc containing fluids, but also fail to teach applicants' solution to the problem. Instead, Heilweil, et al. contain the same conventional teaching of aqueous solutions of various weighting salts without recognizing the advantages of the specific combination of zinc bromide and one or more alkali metal bromides as called for in the subject application (see column 6, lines 38-55). Just as in Heilweil, the Heilweil, et al. patent exemplifies sodium bromide brines and calcium bromide brines, but fails totally to appreciate or hint at the discoveries made by applicants herein.
Pelozo, et al., U.S. Pat. No. 4,615,740 describes a liquid polymer containing fluid incorporating a soluble salt, preferably sodium chloride and/or calcium chloride. (See column 4, lines 26-34.) The examples disclose a series of mono-salt solutions containing NaCl, NaBr, CaCl.sub.2, CaBr.sub.2, and ZnBr.sub.2. Mixed salt solutions of CaCl.sub.2 /CaBr.sub.2 and CaCl.sub.2 /CaBr.sub.2 /ZnBr.sub.2 are also described. (See Tables III and V in columns 5 and 6). The only high density calcium-free solution disclosed is a 19.2 ppg ZnBr.sub.2 solution, but the patent does not recognize that a zinc bromide solution of that density is unsuitable for use as a well completion fluid. Nor does the reference teach that useful calcium-free fluids can only be obtained with a combination of zinc bromide and one or more alkali metal bromides.
It is thus a primary object of the present invention to develop high density completion fluids that may be successfully used in sulfate and/or carbonate-containing wells, in the density range of 15.0 to 20.5 lb/gal.
It is a further object of the invention to develop high density completion fluids having pH values in the range of 1.0 to 5.5 for use in sulfate and/or carbonate containing wells.
An additional object of the invention is to develop high density calcium-free completion fluids for use in carbonate and/or sulfate containing wells which are economical.
Another object of the present invention is to develop high/density calcium-free completion fluids which may also contain corrosion inhibitors and viscosifying agents for downhole applications.
Further objects and uses of the present invention will also be obvious from the following disclosure.